Three-fourths of all rootkits on decade-old OS, says antivirus firm

Machines running the decade-old Windows XP make up a huge reservoir of infected PCs that can spread malware to other systems, a Czech antivirus company said today.

Windows XP computers are infected with rootkits out of proportion to the operating system's market share, according to data released Thursday by Avast Software, which surveyed more than 600,000 Windows PCs.

While XP now accounts for about 58% of all Windows systems in use, 74% of the rootkit infections found by Avast were on XP machines.

XP's share of the infection pie was much larger than Windows 7's, which accounted for only 12% of the malware-plagued machines -- even though the 2009 OS now powers 31% of all Windows PCs.

Rootkits have become an important part of the most sophisticated malware packages, particularly botnets, because they mask the infection from the user, the operating system and most security software. By installing a rootkit, the hacker insures the compromise goes undetected as long as possible, and that the PC remains available to the botnet's controller for nefarious chores, such as sending spam or spreading malware to other machines.

Avast attributed the infection disparity between XP and Windows 7 to a pair of factors: The widespread use of pirated copies of the former and the latter's better security.

"According to our stats, as many as a third of XP users are running SP2 [Service Pack 2] or earlier," said Ondrej Vlcek, the chief technology officer of AVAST, in an interview Thursday. "Millions of people are out of support and their machines are unpatched."

Vlcek assumed that many of the people running XP SP2, which Microsoft stopped supporting with security patches a year ago, have declined to update to the still-supported SP3 because they are running counterfeits.

Although Microsoft serves everyone, even pirates, its monthly security patches and service packs, most security experts believe that users of illegal copies are very hesitant to upgrade or even patch for fear that they'll trigger the black screen and anti-piracy nag notices that Microsoft slaps on screens when it deems a PC is running a counterfeit copy of Windows.